Case wall thickness
"A big contributor to this is cases with quite uneven case wall thickness."
I am often amazed at Lake City Match brass (308) , and how inconsistent the walls are. This is 1970-1980 kinda years.
To check these, I slowly roll them across a flat surface, while listening and watching them. (That's a new one: "Listen to your cases, men!"). Anyway, it is quite obvious when I find a lopsided case: you can hear them speed-up and slow down, and watch as the heavy side comes to rest at the bottom.
I end up culling out about 50% of the cases this way. Not at sure that I shoot them better in a service rifle, but it makes me more confident.